The parallax and proper motion of RX J1856.5-3754 revisited

RX J1856.5-3754, a bright soft X-ray source believed to be the nearest thermally emitting neutron star, has commanded and continues to command intense interest from X-ray missions. One of the main goals is to determine the radius of this neutron star. An integral part of the determination is an accurate parallax. F. M. Walter analyzed Hubble Space Telescope (HST) data and derived a parallax, pi = 16.5 +/- 2.3 mas. Combining this distance with the angular radius derived from blackbody fits to observations of RX J1856.5-3754 with ROSAT, Extreme Ultraviolet Explorer, and HST, Pons et al. derived an observed radius ("radiation radius"), R-infinity = 7 km. This value is smaller than the radii calculated from all proposed equations of state of dense baryonic matter. Here, we have analyzed the same HST data and find pi = 7 +/- 2 mas. We have verified our result using a number of different, independent techniques and find the result to be robust. The implied radius of RX J1856.5-3754 is R-infinity = 15 +/- 6 km, falling squarely in the range of radii, 12-16 km, expected from calculations of neutron star structure for different equations of state. The new distance also implies a younger age for RX J1856.5-3754 of 0.4 Myr, based on its association with the Upper Sco OB association.